1. Field of the Invention
This invention relates to the inhibition of scale formation from oil well brines utilizing a slow release inhibitor composition.
2. Background Information Including Description of Related Art
In the production of crude oil from underground wells, mineral scales such as barium sulfate, strontium sulfate, calcium sulfate and calcium carbonate are often formed on wellbore and equipment surfaces where oil and water containing dissolved salts are coproduced. The formation of scale can slow oil production rate and, at the extreme, stop production completely. An expedient often utilized is to inject or "squeeze" a solution of a scale inhibitor such as a polyphosphonate or a polyacrylic acid into the reservoir rock, often utilizing a brine or water afterflush, and allow the absorbed inhibitor to desorb during fluids production. In practice, however, the desorption process is often found to be quite rapid once production is resumed necessitating frequent shutdowns for additional treatments. This has the effect of substantially reducing the productivity of the well.
Various improvements in the adsorption and desorption of scale inhibitors have been proposed, some of which are disclosed in the following prior art references.
U.S. Pat. No. 3,827,977, issued Aug. 6, 1974 to Miles et al. discloses the in situ deposition in the porous rock formation adjacent to a well bore of a polyvalent metal salt of a polyacrylic acid or partially hydrolyzed polyacrylamide inhibitor by introducing into the porous formation a strongly acidic aqueous solution of a salt of the inhibitor and the polyvalent metal. The acid is partially neutralized by the reservoir liquid causing the polyvalent salt of the inhibitor to phase separate on the porous surfaces.
U.S. Pat. No. 4,602,683, issued Jul. 29, 1986 to Meyers, discloses the inhibition of scale deposition during the operation of an oil well by injecting an inhibitor solution into a subsurface brine producing formation at a higher first pH, and thereafter subjecting the solution to a fluid or substance which reduces the higher first pH to a lower second pH causing precipitation of the scale inhibitor in the formation. Alternatively, the scale inhibitor may be precipitated by injecting a second solution having a pH lower than the second pH into the formation or a substance that lowers the pH of the inhibitor solution to the second pH.
U.S. Pat. No. 5,141,655 issued Aug. 25, 1992 to Hen, discloses a method for the injection of scale inhibitor into reservoir rock which alleviates the problem of too rapid desorption of inhibitor during production, such method comprising injecting into a well reservoir an aqueous acid solution containing a scale inhibitor, multivalent metal ions, and a heat-sensitive substance which decomposes at the temperature of the reservoir liberating an alkaline compound, thus raising the pH of the solution and causing a multivalent metal salt of the scale inhibitor to phase separate out onto the surfaces of reservoir rock formation.
U.S. Pat. No. 5,346,010, issued Sep. 13, 1994 to Adams et al., discloses a process similar to that disclosed in the preceding paragraph, but wherein the inhibitor injection solution also contains a chelating agent for the purpose of preventing iron ions from causing premature hydrolysis of the base generating precursor. The disclosure of this patent also states that an "overflush" may be employed which may contain an additional quantity of the precursor.